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Know the Science: How Medications and Supplements Can Interact

Drugs With a Narrow Therapeutic Range

Having the right amount of certain drugs in your body is crucial. If the amount of the drug is even a little too low or too high, it can cause big problems.

Drugs like these are said to have a “narrow therapeutic range” or “narrow therapeutic index.”

Interactions are of special concern for drugs with a narrow therapeutic range.

Examples of Drugs With a Narrow Therapeutic Range
Carbamazepine (used to prevent seizures)
Cyclosporine (used to prevent organ transplant rejection)
Digoxin (used to treat heart problems)
Levothyroxine (used to treat thyroid problems)
Phenytoin (used to prevent seizures)
Warfarin (an anticoagulant—also called a blood thinner)

If you’re taking a dietary supplement, and your health care provider prescribes a drug with a narrow therapeutic range, what should you do?

Tell me!

Tell your health care provider that you’re taking the supplement and ask the provider what you should do.